They claim Ryanair misinformed authorities about take-off weights on its Boeing 737-800 aircraft by as much as eight tonnes.

If that is true, they avoided paying $21 per flight. Based on the maximum number of flights, that could cost Ryanair $65million.

Online specialist freight magazine Cargo Forwarder reported: “some of the traffic watchers must have become suspicious when comparing the Irish discounter's low MTOM weights with data delivered by other carriers having the same Boeing variant in their fleet.

“This resulted in numerous on-the-spot checks of Ryanair flights to and from Germany, particularly at Ryanair's hub Frankfurt-Hahn. The outcome of these controls seems to have corroborated the aviation authority's concerns over the doubtful weight declaration submitted by the Irish airline.”

Ryan Air said: “Ryanair has a policy of not commenting on rumour or speculation. Ryanair's 737 aircraft operate in full compliance with Boeing's flex-weight programme, as certified and approved by the Irish Aviation Authority (IAA) and we will continue to do so.

“We do not comment on specific flights but we are happy to address any issues or queries raised by EuroControl directly with them and we will continue to do so.”